This invention relates to a circuit for actuating a monostable or bistable inductive device, and more particularly to the actuation of said device upon the opening of a normally-closed switch.
It is sometimes necessary in ac circuits to actuate a monostable or bistable inductive device in response to the opening of a normally-closed switch. This may be accomplished simply and reliably without great expense and without the need of an additional relay by placing the normally-closed switch in parallel with the inductive device and placing this parallel combination in series with a resistor across an appropriate source of ac voltage. However, this solution offers certain disadvantages. For example, when the normally-closed switch opens, the total impedance of the circuit increases. Therefore, the inductive device must have a high sensitivity in order to insure that there will be enough of the source voltage across the inductive element to safely actuate it. Of course, a source voltage having a value several times greater than the rating of the inductive device could be employed. However, this would result in wasted heat dissipated by the series resistor. Further, heat dissipation in the series current limiting resistor in the preactuating mode is certainly undesirable in certain applications; for example, control circuits for submersible pumps used in mines where any heat build-up increases the probability of explosion in areas where combustible gases accumulate.